The invention relates to an operating circuit for a load, which contains a switching transistor that is driven by a driver circuit.
The invention thus relates in particular, but not exclusively, to an operating circuit for lamps.
Operating circuits generally contain at least one switching transistor which, for example in the case of an inverter, connects a load-side connection to a supply potential on a pulsed basis. The switching transistor must be driven accordingly, that is to say it must be supplied with suitable potentials at its control connection, taking into account its function. This also applies in a general form to what are referred to as current-controlled switching transistors, such as bipolar transistors.
The switching transistor is driven by a circuit part which is in general referred to as a driver circuit. This expression, without being restrictive, is used in the following text for any type of driver for the switching transistor.
The driver circuit itself in turn requires appropriate supply potentials. An energy-storage capacitor for storing electrical energy for operation of the driver circuit is frequently used here. This energy-storage capacitor must therefore be supplied with suitable potentials.
Depending on the circumstances of the circuitry for the switching transistor, problems may occur in this case with the provision of the supply potentials for the driver circuit and/or for its energy-storage capacitor. This is particularly true in the case of what are referred to as high switching transistors, one of whose connections is connected to the supply potential of the operating circuit, and whose other connection is connected on the load side (this is what is referred to as a high-side driver circuit). The expression xe2x80x9cload sidexe2x80x9d in this case also means an indirect connection to the load, for example via intermediate inductors, diodes and the like.
The invention is based on the technical problem of specifying an operating circuit for operation of a load having a high switching transistor, in which a driver circuit with an improved supply is used.
For this purpose, the invention is based on a circuit for operation of a load having a switching transistor which is connected on one side (in the above sense) to a supply potential and whose other side is the load side, and whose control connection can at least at times be driven outside the potential range between the supply potential and the potential on the load-side connection of the switching transistor, which circuit also has a driver circuit for driving the control connection, and an energy-storage capacitor for supplying the driver circuit with a supply power; characterized by a pump circuit for charging the energy-storage capacitor from an alternating supply potential which is independent of the potential at the load-side connection, with a series circuit, which connects the connections of the energy storage capacitor, of two rectifier diodes connected in the same direction and with a potential isolation capacitor which is connected to a tap between the rectifier diodes on one side and the alternating supply potential on the other side.
The basic idea of the invention is thus to supply the energy-storage capacitor with an alternating supply potential whose potential can be kept completely independent of the load-side connection of the switching transistor. A potential isolation capacitor is used in the pump circuit, for this purpose. Via this potential isolation capacitor, the alternating supply potential is coupled to a center tap between two series-connected rectifier diodes which are connected in the same direction, thus charging in the same sense an energy-storage capacitor which is connected between those connections of the rectifier diodes which are opposite the center tap. In this case, the potential on the energy-storage capacitor can be related to a reference ground potential which is advantageous for the driver circuit, that is to say in particular also to a potential which fluctuates during operation of the operating circuit. Thus, for example, if an FET (field-effect transistor) is used as a high switching transistor, and its source connection is connected on the load side, then the driver circuit can be operated by the pump circuit with an alternating supply potential whose potential is independent of the source potential on the switching transistor.
Since, for example in the case of inverters, the load-side connection of the switching transistor fluctuates between an (internal) ground potential and the supply potential of the inverter, it is thus possible to supply the driver circuit with a normal supply potential via the pump circuit, even in switching states in which the load-side connection is essentially at the supply potential. This supply potential for the driver circuit may, for example, be the potential which is used for other driver circuits or logic circuits in the operating circuit and in the vicinity of its circuit and, specifically, its magnitude may be considerably less (that is to say closer to ground) than the supply potential for the inverter. The pump circuit thus offers a capability, which is particularly simple in terms of potential and, also in terms of time, can be operated independently of the respective operating phase of the operating circuit, for supplying the energy-storage capacitor.
This avoids, in particular, the necessity (as is known from the prior art) to use quite specific operating phases with advantageous potential relationships for charging the energy-storage capacitor. Reference will be made to the exemplary embodiment, in order to explain this in more detail. Nevertheless, of course, the invention can be used, possibly for other reasons, in a specific manner for operation of the operating circuit, at appropriate times.
One preferred application of the invention is for switching transistors with an FET input, that is to say an actual FET, in particular a MOSFET, or for what are referred to as IGBTs (Insulated Gate Bipolar Transistors). In contrast to pure bipolar transistors, these require relatively high voltages to switch them on, in comparison to the reference ground potential of the transistor.
A protection resistor, which is referred to here as a first resistor, may be provided between the potential isolation capacitor and its alternating supply voltage.
This may be used in particular to protect the output which provides the alternating supply voltage, for example a MOSFET gate, against relatively high currents, which may occur when the reference ground potential of the switching transistor, that is to say in particular the load-side connection, changes its potential quickly, and this sudden potential change across the potential isolation capacitor makes itself evident as a current surge of the output. The protection resistor damps such current surges by virtue of the RC constant that is formed together with the potential isolation capacitor.
Furthermore, potential clamping is preferably provided between said alternating supply voltage and the potential isolation capacitor, in order to make it possible to provide impedance decoupling. This may be done on the one hand by using two series-connected rectifier diodes which are connected in the same direction, and whose center tap is located between the alternating supply potential and the potential isolation capacitor, with a resistor which is referred to here as a second resistor being provided between the center tap and the alternating supply potential. In this case, apart from this, a further protection resistor may also be provided between the center tap and the potential isolation capacitor. On the other hand, a circuit with a zener diode may be used, which clamps a line point between a resistor on the alternating supply potential side (referred to as the second resistor) and the potential isolation capacitor. In this case as well, further protection resistors may be provided between the clamped point and the potential isolation capacitor.
The alternating supply potential may be obtained in widely differing manners. In many cases, an alternating supply potential such as this is already available in any case and can be produced with only a small amount of complexity by means of an unused gate. For example, it is also possible to use drive potentials for other switching transistors, for example in a step-up converter.
If the alternating supply potential for the pump circuit is relatively high, it may be worthwhile protecting the energy-storage capacitor by means of a voltage-limiting element. This may in particular be a zener diode which is connected in parallel with the energy-storage capacitor, and hence in parallel with the series circuit formed by the rectifier diodes. However, the alternating supply potential should not be so high that disturbingly high power losses are produced in such a zener diode. Reference should be made to the exemplary embodiment for illustration.
In general, the invention is also based on operating circuits for any desired loads, but in particular on lamp operating circuits, that is to say electronic ballasts for lamps. Different lamp types are relevant in this case. On the one hand, these may be electronic ballasts for example for halogen incandescent lamps. However, ballasts for gas discharge lamps are particularly preferred. This on the one hand refers to inverters for low-pressure gas discharge lamps.
On the other hand, however, the invention is aimed in particular at ballasts for high-pressure discharge lamps. In the case of ballasts such as these, what are referred to as high switching transistors are used, for example, in full bridge circuits, which contain the high-pressure discharge lamp and apply a largely constant supply potential for the lamp to the lamp, pulsed with a different polarity. Problems which occur as a result of unidirectional operation, such as color shifts, nonuniform electrode wear and the like should thus be avoided. Secondly, in the case of ballasts for high-pressure discharge lamps, step-down converters or other oscillators are frequently used to produce an alternating voltage supply which is required for a lamp inductor, and these likewise contain high transistors.
These two applications can also occur combined with one another in ballasts for high-pressure discharge lamps.